In a letter containing more than 25 sworn affidavits from Minnesota patients and medical providers, Swanson requested the federal Centers for Medicare and Medicaid Services (CMS) to probe whether Humana violated federal regulations by wrongfully handling claims in the state. Federal law bars states from regulating Medicare Advantage plan benefits and delegates that authority to CMS.

"Medical bills that aren't covered or processed properly can hit senior citizens hard in the pocketbook," Swanson said in a prepared statement.

Medicare Advantage plans are private health insurance policies approved by CMS but sold and administered by private insurance companies as an alternative to traditional Medicare fee-for-service coverage. Minnesota has the highest number of Medicare Advantage plan enrollees per capita in the nation, according to a Kaiser Family Foundation report.

In her letter, Swanson said Minnesota patients and providers accused Humana of:

-- Denying claims involving Medicare-covered services.

-- Overcharging for co-payments and co-insurance.

-- Failing to adequately disclose what providers are in network and update its network provider directories.

Humana spokeswoman Kate Marx said neither the attorney general nor CMS had notified the company of the complaint.

"We take this very seriously and are working to identify the facts," Marx said in an email. "We have been successfully serving the people of Minnesota for more than 10 years, and more than 100,000 Minnesotans belong to Humana plans."

A publicly traded for-profit insurance company, Humana is one of the biggest insurers in the Medicare Advantage market in the nation, offering plans in all 50 states, the attorney general's office said. Shares of Humana lost 2.8 percent Friday to close at $91.23.